Project Summary/Abstract Cigarette smoking is the leading cause of preventable death in the United States. Given that relapse to smoking is the most likely outcome of any given quit attempt, there is a pressing need to expand treatment options for tobacco use disorder (TUD) to improve sustained abstinence and prevent relapse to smoking. Preclinical literature highlights the role of glutamate in substance use disorders, suggesting that new targets for pharmacotherapy should focus on the restoration of glutamatergic function. One glutamatergic agent that has garnered significant attention recently is N-acetylcysteine (NAC), which is a safe and well tolerated medication that has shown early promise as a pharmacotherapy for substance use disorders broadly, including TUD. Preliminary clinical studies have demonstrated NAC's safety, tolerability, and potential efficacy for promoting abstinence from several drugs of abuse. However, results from randomized clinical trials have been mixed. As a pharmacotherapy for TUD, work conducted with NAC up to this point has been limited to pilot studies, which have been generally encouraging, but also somewhat mixed. Based on the literature, two gaps in knowledge regarding NAC for TUD have emerged: 1) is NAC effective in initiating abstinence; and 2) is NAC effective for relapse prevention, following a period of abstinence (or both). This proposed R34 will address these gaps by evaluating NAC as a pharmacotherapy for TUD. In order to rigorously test NAC for both initial cessation and relapse prevention, participants will be contingently reinforced for abstinence from smoking for three days while undergoing NAC (2400 mg per day) or matched placebo treatment. Participants will verify abstinence through breath carbon monoxide (CO) samples taken remotely twice per day. Following three days of reinforced abstinence, participants will complete an 8-week treatment intervention, in which they will continue to take NAC or placebo. This study will examine the efficacy of NAC, compared to placebo, in helping smokers achieve three days of continuous abstinence (Aim #1), examine the time to relapse over the 8-week intervention between NAC and placebo groups, among those who were abstinent for the 3-day period (Aim #2), and assess 7-day point prevalence abstinence at the 8-week end-of-treatment study visit (Aim #3). Results will inform the treatment literature and provide a better understanding of how NAC is best used as a smoking cessation pharmacotherapy to achieve the best treatment outcomes for smokers. Mixed results from the clinical literature on NAC's efficacy suggest that the knowledge to be gained from the proposed application is essential prior to implementing NAC in larger randomized controlled trials or for use in a clinical population. Findings from this trial may have notable impact, as NAC could be used as part of combination treatment with abstinence- targeted pharmacotherapies or behavioral interventions, in addition to providing the preliminary data necessary for a fully-powered randomized controlled trial (R01) of NAC for TUD.